1. Field of the Invention
The present invention pertains to the synthesis of propylene oxide from hydrogen peroxide in the presence of a titanium silicalite catalyst, or from hydrogen/oxygen mixtures in the presence of a commercially viable noble metal-treated titanium silicalite catalyst.
2. Background Art
Propylene oxide is an important alkylene oxide of commerce. Large amounts of propylene oxide are used, inter alia, for the preparation of nonionic polyether surfactants and of polyether polyols for manufacture of polyesters and other polymers, but in particular for polyols for manufacture of polyurethanes, the latter including both homopolymeric polyoxypropylene polyols and copolymeric polyols prepared using other alkylene oxides, particularly ethylene oxide, in addition to propylene oxide. Propylene oxide also has numerous other uses in organic synthesis.
Older methods of propylene oxide production employed the “epichlorohydrin” process, a process which employs toxic chlorine, and generates numerous chlorine-containing byproducts which present environmental concerns but is still in use today. Several “coproduct” processes have been disclosed and/or implemented. In one major commercial process, indirect oxidation of propylene by ethylbenzene oxidation products to form propylene oxide yields styrene as a major coproduct. In both this as well as other coproduct processes, the economic value of the coproduct has a great effect on the overall process economics. The value of the coproducts may at times by undesirably low. Thus, it is desirable to employ a process which does not rely on coproduct economics to produce propylene oxide.
A “direct” method of propylene oxide production has long been sought. In such “direct” methods, propylene oxide is produced by oxidation of propylene with oxygen or with a “simple” oxidizing precursor such as hydrogen peroxide, without the use of significant amounts of co-reactants and concomitant generation of co-products from these co-reactants. Even though a great deal of research has been expended in these efforts, “direct” production of propylene oxide has not heretofore become a commercial reality.
In U.S. Pat. No. 5,401,486, it is disclosed that propylene oxide may be produced by the “direct” oxidation of propylene by hydrogen peroxide in the presence of a titanium silicalite catalyst, citing EP A-100,118. However, the latter indicates that the principle products of olefin oxidation are ethers, with olefin oxides prepared only in minor amounts. The titanium silicalite useful in such processes, despite the relatively low yield of olefin oxides, has been generally acknowledged by the art to be limited to exceptionally small titanium silicalite crystals substantially free of the anatase form of titanium silicalite. These crystals are about 0.2 μm or less in size. Since the catalysts are heterogenous catalysts, use of larger particle size catalysts, with their decreased surface area, should result in a considerable decrease of activity and product yield in the oxidation of alkenes. For example, the rate of oxidation of linear alkanols employing titanium silicalites has been shown to be reduced as crystal size of the titanium silicalite is increased. See, e.g., “Oxidation of Linear Alcohols with Hydrogen Peroxide Over Titanium Silicalite 1,” A. Van der Pol et al., Schuit Institute of Catalysis, Eindhoven University of Technology, APPL. CATAL. A., 106(1) 97-113 (1993), which indicates that a particle size less than 0.2 microns is necessary to obtain maximum catalyst activity. See also U.S. Pat. No. 6,106,803, which indicates that high catalytic activity can only be obtained with small primary crystals of titanium silicalite. The U.S. Pat. No. 6,106,803 patentees teach preparing small primary crystals and using these crystals to form granulates of larger size by spray-drying. These and other publications have discouraged investigation of the use of large titanium silicalite crystals.
Other references which relate more directly to olefin epoxidation indicate that selectivity and hydrogen peroxide conversion efficiency are decreased by the presence of anatase in the titanium silicalite catalyst. See, e.g., “Preparation of TS-1 Zeolite Suitable for Catalyzing the Epoxidation of Propylene,” H. Gao et al., Shanghai Research Institute of Petrochemical Technology, Shanghai, Peoples Republic of China, Shiyou Xuebao, Shiyou Jiagong (2000) 16 (3), p. 79-84; and “Synthesis and Physicochemical Properties of Zeolites Containing Framework Titanium,” C. Dartt et al., California Institute of Technology, Pasadena, Calif., MICROPOROUS MATTER, 2 (5) p. 425-437 (1994).
However, use of small titanium crystals, e.g. those having mean sizes of about 0.2 μm or less is highly problematic in commercial epoxidation of alkenes. In fixed bed processes, the small particle size creates an enormous pressure drop which renders the process unworkable, while in slurry processes, separation of the catalyst from the liquid reactor contents is extremely difficult. Moreover, due to the attrition of particulate catalysts in commercially useful reactors, the particle size decreases over time, eventually plugging filters designed to recover and recirculate catalyst back to the reactor. As a result, although the catalyst activity of small particle size catalysts is reasonably high, a commercial process employing such catalysts is not practical.
To improve the longevity of the olefin epoxidation process, small titanium silicalite crystals have been conglomerated into formed particles of larger size through the use of binders, as taught, for example, by U.S. Pat. Nos. 5,500,199 and 6,106,803. However, such conglomerated catalysts suffer from several defects. The binder, though porous, will necessarily obscure portions of the zeolite structure, thus effectively removing such portions as catalytic sites in the reaction. Unless the binder has high adhesive and cohesive strength, the formed particles will again be subject to attrition as the conglomerates break apart. Increasing binder content can minimize attrition, although the likelihood of obscuring catalytic sites is then higher. Moreover, the catalyst is essentially “diluted” by the binder on a weight/weight basis, thus requiring greater amounts of catalyst for the same epoxide production rate.
It would be desirable to directly epoxidize propylene in the presence of large titanium silicalite crystals which exhibit high activity, low attrition rates, and freedom from use of binders, and which do not cause rapid plugging of catalyst filter elements.